1. Field of the Invention
This invention relates generally to dilatometers and, more particularly, to dilatometers having a hermetically sealed housing for both the sensor and the specimen. Such unitary housing permits operation of the dilatometer under vacuum or in a protective gas atmosphere and provides the operator with convenient access to the sensor for replacement, calibration or other necessary adjustments, as well as enabling him to quickly and easily replace specimens.
2. Description of the Prior Art
Dilatometers are analytical instruments that respond to the linear thermal expansion or contraction of solids. Typically, these instruments have a variable temperature furnace in which the test specimen rests between a flat surface on a stationary object and an opposing flat surface on a movable object, such as a ceramic pushrod, that extends outside the furnace. Temperature induced changes in the length of the specimen are transmitted through the rod to a mechanical, optical or electrical system for amplifying and measuring that change. These instruments can be used to make precise measurements of changes in length resulting from small temperature changes or to plot variations in the rate of linear expansion or contraction over a broad temperature range.
Among the least sophisticated dilatometers in common use are those in which the push rod is coupled to a dial gauge and the dilation of a specimen is read directly from that gauge. Such dial gauge dilatometers are simple to use and inexpensive, but generally are suitable only for low to moderate temperature applications that do not demand great precision.
U.S. Pat. No. 3,680,357 describes a far more precise type of dilatometer in which the dilation sensor is a linear variable differential transformer which translates specimen dilation into electrical signals that can readily be amplified and recorded. In such sensor, the core floats freely in the coil and each of these elements is separately supported at its ends by a pair of compound cantilevered springs. These springs permit independent and frictionless axial movement of the suspended element, but restrain radial or transverse movement. This independent and frictionless axial mobility of the core and coil facilitates calibration of the sensor and renders it extremely sensitive to minute changes in specimen length, thereby making possible exceptionally accurate measurements of thermally induced expansion or contraction.
Such dilatometers have been used for a wide variety of purposes. For example, those which have a single pushrod coupled to the core or coil of the linear variable differential dilatometer, as described in U.S. Pat. No. 3,805,589, are used in the steel industry to detect phase changes occurring when heat softened steel is cooled and to study the effect of different cooling rates on the physical properties of the finished product.
Such dilatometers which have separate pushrods coupled to the core and coil of the linear variable differential transformer, as described in U.S. Pat. No. 3,898,836, are widely used for making differential measurements on different specimens. These differential measurements are invaluable in studying the compatibility under changing temperature conditions of different materials which are bonded together or are in close contact; e.g., metal to glass, enamel to substrates, thin film deposits in microcircuits or metal or plastic fillings in natural teeth.
Dilatometers employing a linear variable differential transformer sensor also are available in which one or both of the core and coil are supported by linear ball bearings which permit low friction axial movement of the supported member while rigidly restricting axial movement, as illustrated in copending U.S. application Ser. No. 538,180, filed Oct. 3, 1983, which now is U.S. Pat. No. 4,521,119, issued June 4, 1985. Dilatometers having these sensors are particularly useful for making measurements on specimens that exhibit extremely large dimensional changes or for applications requiring the imposition of a constant force on the specimen.
Regardless of the dilation sensor that is employed, the temperature ranges that must be investigated for many applications are sufficiently high that it becomes necessary to protect both the specimen and the sensor from oxidation. It also is desirable for optimum accuracy of the sensor to minimize temperature differentials across the sensor and its adjacent metal supports and peripherals; e.g., the micrometer (which is used for calibration) and the sliding carriage (which accomodates specimens of differing length) that are illustrated in FIG. 1 of U.S. Pat. No. 3,680,357. As exemplified by FIG. 6 of that Patent, protection against oxidation and undesirable temperature differentials commonly is accomplished by placing the sensor, along with its adjacent metal supports and peripherals, within a metal box-like enclosure that is clamped with a hermetic seal on one side of a temperature stabilized radiation shield that is fixed to the instrument base. The push rod or rods extend through a small opening in the shield to abut the specimen or specimens within a communicating tubular enclosure that is similarly clamped and sealed on the other side of the shield and that extends into the furnace. Evacuation of the communicating enclosures minimizes the possibility of oxidation within the enclosures and, because it effectively blocks heat transfer to the sensor by both radiation and convection, measurements can be made with great precision at extremely high temperatures.
These enclosures do, however, make it difficult for the operator to visually observe the sensor, which is often desirable and is essential when using a dial gauge. It also hinders the operator when he must insert a specimen and calibrate the sensor, as the furnace first must be moved or opened to provide access to the tubular enclosure and both enclosures must then be unclamped and removed separately. This often is awkwark and is potentially dangerous when the furnace and tubular enclosure are still hot from a previous measurement. The time and effort required to change sensors also effectively precludes the use of a single instrument for multiple applications that require the use of different specialized sensors.
It is an object of this invention to provide a dilatometer having an improved hermetically sealed housing for the dilatometer sensor, pushrod and specimen holder, which includes an elongated protective tube enclosure for the portion of the specimen holder that is inserted in an electric tube furnace. It is a specific object of this invention to provide such housing which is light weight and compact and which can easily and quickly be withdrawn as a unit from a stationary or unopened furnace to facilitate the rapid installation of similar housings having specialized sensors or tubular enclosures designed for different applications. It also is a specific object of this invention to provide such housing from which the sensor, push rod and specimen holder can be quickly removed as a unit for changing specimens, without the necessity of withdrawing the protective tube from the furnace or realigning it after reinsertion. Another specific object is to protect the contents of such housing from oxidation by evacuating the housing and by providing a flow of protective gas across the specimen. A further object of this invention is to provide such dilatometer having an improved dilation sensor design which minimizes temperature differentials across that sensor.
It has been found that these objects and other advantages, which will be apparent from this specification, are achieved by the invention described below.
Broadly, my invention is a compact unitary housing for a dilatometer sensor, pushrod and specimen holder which protects its contents from furnace heat, which conveniently can be handled as a unit and from which these contents can easily be withdrawn as a unit.
One aspect of this invention is such housing for a dilation sensor, pushrod and specimen holder of a dilatometer comprising
(a) a sleeve for encircling said sensor,
(b) a cover closing one end of said sleeve,
(c) a sensor attachment beam secured to said cover and extending into said sleeve,
(d) a ring shaped plate closing the other end of said sleeve and being demountably held to said attachment beam to lock said cover to said sleeve, and
(e) a protective tube for said specimen holder, said protective tube being closed at one end and being coupled at its open end to said plate distal to said sleeve, the aperture of said plate and the bore of said protective tube being aligned.
A specific application of this aspect of the invention is the use of such housing in an encased dilatometer comprising
(a) a dilation sensor mounted on an elongated attachment beam,
(b) a tubular fixed specimen holder having an open end and a closed end that is internally adapted to abut a flat surface of a specimen, said specimen holder being held adjacent its open end by a clamp mounted on said attachment beam,
(c) a movable pushrod that is coupled at one end to said sensor and at the other end is adapted to abut a second parallel flat surface on said specimen, said pushrod extending into the open end of said tubular fixed specimen holder,
(d) a tubular sleeve encircling said attachment beam, sensor and clamp,
(e) a cover closing and hermetically sealing the end of said sleeve distal to said clamp, said cover being attached to an end of said attachment beam,
(f) a ring shaped plate closing and hermetically sealing the other end of said sleeve and being demountably coupled to the other end of said attachment beam to adjustably lock said cover to said sleeve, said plate containing a vacuum port and having a connecting tube extending from the rim of its aperture distal to said sleeve and terminating in a first flange at its open end and
(g) a protective tube having a closed end and a second flange at its open end which is demountably clamped and hermetically sealed to said first flange, said specimen holder extending loosely through said aperture and connecting tube into the open end of said protective tube.
Another aspect of this invention is a compact design for a dilation sensor for a dilatometer that minimizes temperature differentials across the sensor and that advantageously is used in the aforementioned housing. This improved sensor comprises a linear variable differential transformer having a core and coil that are independently axially movable and a calibrating micrometer, said micrometer being positioned adjacent to and abreast of said linear variable differential transformer and bearing on a lateral extension of said core or coil.